Add docker override file name to .gitignore

[miminari]

We need to override the docker file to build a local environment with docker on m1 Mac, so I add that filename to gitignore.
And I also add .DS_Store files are created by the Mac OS X operating system.

Props @ryelle

How to test the changes in this Pull Request:

1. Checkout new branch.
2. Add a docker-compose.override.yaml and a .DS_Store file.
3. Git add, git status, you can check to ignore these files.

[ryelle]

@miminari Thanks again for the PR! I've pushed two commits to this branch. One to move the docker override file to the "Docker" section. The other change removes the .DS_Store from the gitignore. In the comments to this PR #139 it was decided that system files like that should be ignored globally, not in the project configuration.

[miminari]

@ryelle Thank you for commit and merge!

[2ndkauboy]

Haha, great that this was added. In a different issue I experienced today at WCUS Contributor Day, I also had to add such a file :)

@miminari what overwrite is necessary for an M1? I was about to add an example.docker-compose.override.yml and might add your customization into it as well.

[miminari]

@2ndkauboy Hi, I just added same codes written on this docs.

https://github.com/WordPress/wordcamp.org/blob/production/.docker/readme.md

'''
Using an Apple ARM64 (Silicon) chip? You may see failed to solve: rpc error: code = Unknown desc =... or failed to solve: mysql:5.7-debian: no match for platform in manifest after running either of commands above. Try adding platform: linux/amd64 to both wordcamp.test and wordcamp.db in docker-compose.yaml. This will instruct Docker to create an image based on the linux/amd64 architecture instead of linux/arm64, i.e.
wordcamp.test:
build:
context: .docker
dockerfile: Dockerfile.php-fpm
platform: linux/amd64

wordcamp.db:
build:
context: .docker
dockerfile: Dockerfile.mysql
platform: linux/amd64
'''